1. Field of the Invention
This invention relates to a viewfinder for a photographic camera, and more particularly to an Albada finder for a compact camera and the like.
2. Description of the Prior Art
There has been known an Albada finder, a virtual image viewfinder for a photographic camera. The Albada finder comprises an objective lens group on the object side and an ocular lens group on the eye side. One face of a lens element in the objective lens group is concave toward the ocular lens group, and the concave face is coated with a multi-layer film of Al, MgF.sub.2, ZnS, SiO.sub.2 and the like deposited thereon to form a semitransparent mirror. A field frame is formed on a face of a lens element in the ocular lens group facing the objective lens group by depositing thereon metal material such as Al, Ag and the like. Light passing through the objective lens group is reflected by the field frame, is further reflected by the semitransparent mirror and impinges on the eye through the ocular lens group, whereby an image of the field frame is viewed.
If an apparent field of view is narrow when an object is viewed through the finder, the user feels pressure and will have a bad impression on the camera. On the other hand, if the apparent field of view is too wide, it becomes difficult to view the whole field at one time and the user must move the pupil. Accordingly, the apparent field of view should have such an angle that the user will not feel pressure and not have to move the pupil.
Further it is desired that the effective diameter of the eyepiece is large in order to increase the degree of freedom in the position of the pupil, thereby giving the user a feeling of release. Recently as compact cameras have come into wide use, plastic materials come to be wide used as the material for making viewfinders due to their suitability for mass production. Since plastic materials are low in refractive index as compared with glass, aspheric surfaces must be often used in order to correct aberrations. However, when a face in the ocular lens group, especially, the face closest to the eye in the ocular lens group is aspherical, stability of an image viewed through the finder is remarkably deteriorated in response to movement of the pupil since only the optical axis can be an axis of rotational symmetry in the aspheric lenses, which results in fatigue of the user. In the case of a spherical lens, all lines normal to the spherical face can be an axis of rotational symmetry and accordingly spherical lenses are free from such a problem. Thus, it is preferred that the ocular lens group has only spherical faces including a planar face.
In viewfinders, aberrations must be corrected with respect to both an image viewed through the finder (will be referred to as "field system", hereinbelow) and an image of the field frame (will be referred to as "field frame system", hereinbelow). In the Albada finder, when the effective aperture of the ocular lens group is enlarged, aberrations with respect to the field frame system provided in the ocular lens group becomes difficult. That is, though aberrations with respect to the field system can be corrected by providing an aspherical face in the objective lens group, an attempt to correct aberrations with respect to the field frame system by providing an aspherical face in the ocular lens group results in fatigue of the user and deterioration in the value of the camera as described above.